How to build a lung

Tissue engineering, a field that combines cell biology, engineering, and materials science to manufacture tissues – and more recently even whole organs – to replace those lost to injury or illness, must be one of the most exciting areas in modern medicine.

Since the earliest reports about a mouse with a human ear growing on its back over a decade ago progress in this field has been rapid. Now science writer Ed Yong has written an excellent article on his Not Exactly Rocket Science blog about how a team of scientists at led by Laura Niklason Yale University are moving on from the trachea to a far more complex part of the respiratory system – the lung – and successfully transplanted it into rats. As Ed points out, this technology needs to be improved significantly before it can be attempted in humans, and further research in rats is underway to do just that. This work will take time, and as it progresses will almost certainly require studies in larger animals such as pigs whose lungs are closer to ours in size and structure than those of rats. Human trials are not expected for perhaps a decade or more.

Ed Yong was not the only one to note the importance of this research, the journal Science, in which the study was published (1), have included an interview with Laura Niklason in their latest podcast.

How to build a lung. Courtesy of Laura Niklason and Thomas Petersen.

Laura Niklason’s past record certainly gives cause for optimism. In 1999 they published a paper describing how they engineered arteries in vitro that supported blood flow when transplanted into pigs, an animal whose cardiovascular system is a valuable model for our own, and determined that a culture technique that mimics the pulsating arterial blood flow produced stronger and safer engineered arteries. Following a decade of refinement through in vitro tissue culture and animal research the artery is expected to enter human clinical trials next year.

This wasn’t the only exciting lung-related research to be published in Science this week. Scientists at Harvard University have used microfluidics to re-create the interface between the alveoli and capillaries (2) in the lung where exchange of oxygen and other gasses takes place. The response of this “Lab-on-a-chip” model to bacterial infection and inflammatory signals was similar to that seen in previous animal studies.

This technology represents huge advance over existing in vitro models of the lung; which, in addition to being a very promising research tool in its own right, has the potential to reduce the number of animals used in testing the effect of new drugs or toxins on lung function. Eventually an improved version, perhaps combined with chips that simulate other tissue types, might replace animal use in the evaluation of toxicity in the lung entirely, though that goal is still years of dedicated research away. Lab-on-a–chip technologies such as this that can integrate several cell types into a system that mimics real tissues in vivo are a great example of the 3Rs in action.

One area the Harvard scientists were particularly interested in is using this lab-on–a-chip to evaluate the potential toxicity of nanoparticles, since existing in-vitro cell and tissue culture technologies are not adequate for this task, and using rodents is slow and expensive. Since nanoparticles are becoming increasingly common in daily life there is an urgent need to develop ways to rapidly assess their safety before humans and animals are exposed to them. So they examined how their lab-on–a-chip responded to a variety of nanoparticles, and then compared the results to those of parallel studies performed on the lungs of mice.

A key question was whether inhaled nanoparticles can cross into the bloodstream, several animal studies indicate that they can while in vitro studies suggest otherwise, though as mentioned the relevance of these in vitro methods has been questioned. With the new technology the results were in close agreement, the nanoparticles can cross into the bloodstream. This demonstration indicates that the lab-on-a-chip may provide a suitable platform for future evaluation of aspects of nanoparticle toxicity, as part of new pathways for the evaluation of chemical safety that use as few animals as possible.

So, all in all it is a very great week for building lungs in Science, one to which animal research made a huge contribution.

In my rush to finish the above post I forgot to mention another advance in the use of decellularized scaffold and in vitro cell repopulation approach to tissue engineering, scientists at Harvard Medical School produced artificial livers that appeared to function almost as well as normal tissue when transplanted into rats and connected to their blood supply . In the research paper published online in Nature Medicine the authors stress that the artificial liver needs further development before human transplants can be contemplated, but this is further evidence of just how quickly progress is being made in the field of complex tissue engineering.